Bar codes are parallel sequences of bars which are formed two-dimensionally but mostly vary only one-dimensionally, the thickness and spacing thereof coding information. They are present today in many economic sectors for the identification of objects and consumer goods. As a rule, a bar code represents merchandise-related, machine-readable information which codes for example an article number, indication of origin and/or the price of the merchandise. In particular, it is possible to form such bar codes as security features, for example by using fluorescent or luminescent dyes, and to apply them to documents in order to code security-relevant information, such as the denomination of a bank note, the name of the owner or the like.
Scanning apparatuses for the automatic scanning or reading of such bar codes are widely known in the prior art. Such scanning apparatuses comprise at least one light source, frequently a light emitting diode or laser diode, which illuminates the area of the document or object provided with a bar code, a photoelectric detector arrangement which recaptures the light after the scanning of the bar code and transforms it into an electric signal, and an evaluation device which determines the information of the bar code on the basis of the received electric signal.
DE 102 12 734 A1 discloses a scanning apparatus with a laser diode, a mirror system and a detector arrangement which scans a bar code via the mirror system along a measuring line perpendicular to the bars of the bar code. If the bar code has artifacts in the area of said measuring line given by the scanning apparatus, or if the bar code interferes unfavorably with the background pattern of the document in the scanned places, a reliable recognition of the bar code pattern is difficult and the reliable decoding of the information is jeopardized.
DE 199 24 750 A1 discloses a scanning apparatus for bar codes with two light sources and two detector units for scanning a bar code along two different, substantially parallel measuring lines. For this purpose, the document bearing the bar code is guided past the scanning arrangement. However, DE 199 24 750 A1 relates to the scanning of two-dimensional bar codes which have a variable bar pattern in the direction of the measuring line but are divided into two separate partial bars in the direction perpendicular thereto, so that a different black and white pattern is scanned along each of the two measuring lines.
This permits the amount of coded information to be doubled at an equal reading rate in comparison to the apparatus of DE 102 12 734 A1. However, the problem of the artifacts is not solved and the reliability of scanning is not improved in this respect.
An increased reliability of scanning is obtained, however, when the apparatus proposed in DE 102 12 734 A1 is used to scan a conventional one-dimensionally varying bar code and the two results are subsequently matched by an evaluation device. However, such multitrack scanning requires a measurement setup consisting of at least one light source and detector for each measuring track.
The redundancy-based gain in reliability thus comes at a cost which increases linearly with the number of measuring tracks. Furthermore, this principle of hardware redundancy cannot be readily implemented in compact and light scanning apparatuses due to the required space.